How to Building a pay per transaction site?

I have to build a comparison shopping web site,

where I will be paid by merchants on per successuful transaction basis.



what are the ways to track whether a successful transaction was initiated from my site?



Are open source solutions/templates available for the same?



Does any CMS(for e.g. Joomla) do it?



In summary, let me know the free/paid solutions of the above problem.



thanks in advance.